Semiconductor laser devices have been widely used in optical disc systems and optical communications (see, e.g., Japanese patent publication No. JP-B-3080312, JP-A-2002-100830 and JP-A-2004-296903). These semiconductor laser devices include a cavity to generate a laser beam. The cavity has a front facet formed on one end thereof to emit the laser beam and has a rear facet formed on the other end. An insulating film, or coating film, is formed on each facet to reduce the operating current of the semiconductor laser, to prevent return of light, and to increase the output power.
Generally, in high power semiconductor lasers, a low reflectance coating film is formed on the front facet, and a high reflectance coating film is formed on the rear facet. Specifically, the reflectance of the coating film on the rear facet is typically 60% or higher, preferably 80% or higher. The reflectance of the coating film on the front facet, on the other hand, is determined based on the required characteristics of the semiconductor laser. (Lower reflectance does not necessarily guarantee higher laser performance.) For example, semiconductor lasers for exciting a fiber amplifier used in combination with a fiber grating employ a coating film having a reflectance of approximately 0.01-3%. General high power semiconductor lasers, on the other hand, employ a coating film having a reflectance of approximately 3-7%, or a coating film having a reflectance of approximately 7-20% when it is necessary to prevent return of light.
The coating films protect the facets, that is, they function as passivation films for the semiconductor interfaces (see, e.g., Japanese Patent Publication No. JP-A-2004-296903 noted above). However, depending on the coating method used, the facets may be damaged, resulting in degraded characteristics of the semiconductor laser. Therefore, it is important to select the appropriate coating method.
Japanese patent publication No. JP-A-1984-232477 discloses that an anodic oxide film may be formed to protect a GaInAsP semiconductor. In the case of conventional gallium nitride semiconductor lasers, on the other hand, it is known that the coating films tend to delaminate or peel off resulting in degradation of the facet surfaces. To prevent this, a structure has been proposed in which a thin film of a metal or an oxide thereof is inserted between a coating film and the underlying semiconductor as an adhesive layer (see, e.g., JP-A-2002-335053). Further, Japanese patent publication No. JP-A-2006-203162 proposes a structure in which a thin layer of hexagonal crystal is inserted between a coating film and the underlying semiconductor as an adhesive layer.
For reference, followings are prior art Japanese patent publications.
No. JP-B-3080312,
No. JP-A-2002-100830
No. JP-A-2004-296903
No. JP-A-1984-232477
No. JP-A-2006-203162
However, metal adhesive layers have low insulating properties (and may cause shorting). Further, a metal oxide film is difficult to form to a controlled thickness by sputtering. Further, although a coating film is preferably formed of amorphous or a single crystal, it is difficult to form a hexagonal single crystal.
The present invention has been devised to solve the above problems. It is, therefore, an object of the present invention to provide a semiconductor laser device in which an easy-to-form adhesive layer is provided between a coating film and the underlying semiconductor.
Other objects and advantages of the present invention will become apparent from the following description.